The present invention relates to an optical head for focusing a light beam for reading or recording information onto or from an information recording medium such as an optical disk, and more particularly, to an improvement in a photo detector for an optical head for detecting a defocusing state of a light beam.
Various optical information recording/reproduction systems have been recently devised which optically write and read informtion onto and from an information recording medium (to be referred to as an optical disk hereinafter). For example, information recording/reproduction systems are known which are used for a recording medium for reproduction only such as a CD (compact disk) type of DAD or a video disk, an image file, a still image file, a COM (computer output memory) or the like. In such systems, information is recorded by establishing a state change such as forming a hole (pit) in a recording layer by a focused light beam. In other information recording/reproduction systems, information is optically written on or read from an information recording medium for recording/reproduction or from an erasable information recording medium (to be referred to as an optical disk hereinafter). In these systems, in both the write and read modes, a light beam must be always focused on the optical disk. In view of this requirement, the optical head frequently has a focus detecting system for detecting the state of focus of the light beam. Various such focus detecting systems have been proposed. For example, U.S. Ser. No. 399,873, filed on July 19, 1982 (now U.S. Pat. No. 4,521,680) and corresponding EPC application No. 82106508.3, filed on July 19, 1982 disclose a focus detecting system adopting the so-called knife edge method.
The photo signals generated by a photo detecting unit of such a focus detection system are processed by a focusing signal generator of the system and are converted into a defocus signal or into a just in-focus signal. These signals are supplied to a voice coil for driving the objective lens so that the objective lens is kept in the just in-focusing state. The focus detecting system must have a satisfactory sensitivity and stability. In a conventional optical head, if a focus detecting system is sensitive, the focus servo system is operated less stably. The conventional optical head is known to have other problems to be discussed below.
(1) Noise is generated in the focus servo loop or system due to optical or electrical disturbance. Due to this noise, the objective lens may bump against the optical disk or may be excessively separated therefrom.
(2) When there is a defect in the light reflecting layer of the optical disk or the optical path of the optical system of the focus detecting system is shielded for any reason, the laser beam may not become incident on the photo detecting unit and the objective lens may be overdriven.
(3) In a focus detecting system of a certain type, when the defocusing displacement is great, the level of the defocus signal is decreased. Then, the focus servo system may not operate normally.